Biaxially oriented polyester film, particularly polyethylene terephthalate (PET) is widely used as support for photographic products. The surface of such PET film is hydrophobic and is not very receptive to coatings of hydrophilic layers such as the gelatinous photographic layers. It is often necessary to apply to one or both sides of PET film a primer coating which adheres to the film and is receptive to other overcoatings.
Many different families of polymers have been suggested for preparing a primer layer on a PET film, particularly where the film is to be suitable for photographic use. A pioneering patent, U.S. Pat. No. 2,627,088, describes a primer composition comprising a terpolymer, with polyvinylidene chloride (PCDC) as major component, which gives good adhesion both to PET and to the hydrophilic gelatinous overcoated layer. This discovery generated many applications for the good properties of receptivity provided by the PVDC copolymers. It is well understood in the photographic art that a substrate for a photo-layer application needs performances in areas other than just adhesion: good wetability of the surface is needed to allow good and fast coating of a subbing layer; and good antistatic properties are desirable to avoid problems connected with charge accumulation on the film (dust attraction and electrical spark emission which are recorded by the sensitive film). These other performance characteristics are usually achieved by coating a second suitable layer called a subbing layer. In addition to the above cited quality performances needed in the film, the economic performances must be considered. The most important economic consideration for prior art films is related to the impossibility of recycling the scraps containing PVDC primer generated during the manufacturing of the film. The chloride containing polymer primer is not stable at the high temperature of the PET extrusion. Scrap containing PVDC could not be reextruded because its decomposition would impart an undesirable yellow discoloration to the finished oriented film. A process for washing off the PVDC primer to the PET primed film is done and this results in an additional high cost. Moreover, during the manufacturing of the film the edge trim of this primed film often is sent directly to the extrudor blend: the final result is a continuous increase in the yellowness of the manufactured film.
The good adhesion of PVDC polymer to the PET is essentially due to the intermingling of two polymers at the interface during the heat setting (temperatures higher than the melting points) operation. In fact the PVDC polymer doesn't adhere well to the PET when coated without heat setting. The adhesion of the gelatin layer to the PVDC layer is due to the possibility of covalent bond formation between the electrophilic carbon atoms activated by the couple of contiguous electronegative chlorine and the nucleophilic amino groups of gelatin: ##STR1## When a polymer different from PVDC is used for the primer, it must contain a gelatin reactive group in the side-chain or (this is the case for example of a unreactive styrene-butadiene primer) it must contain an additional adhesion promoter partner which is reactive with gelatin. The most common are the so-called "gelatin hardners" i.e. bifunctional reactive compounds crosslinking the gelatin macromolecules: melamine-formaldehyde is claimed in U.S. Pat. Nos. 4,123,278, 3,819,771, U.K. Pat. No. 1,540,067; dichlorotriazine salt in U.S. Pat. No. 4,407,939 and dimethylol urea in U.S. Pat. No. 4,424,273.
Within the polymeric primers containing reactive groups different form chlorine (PVDC), the polymers containing epoxy groups reactive with gelatin are diffusely claimed (U.S. Pat. Nos. 3,645,740, 4,098,952, 4,128,426 and 4,328,283, U.K. Pat. Nos. 1,593,343, 2,037,792 and 2,046,626).
Generally speaking, the epoxy groups are not reactive enough and powerful enough to assure good adhesion to the PET surface and to the gelatin layer without an additional physical treatment of the surface.
No PET film has been found on the market for photographic applications having auxiliary priming layers containing epoxy groups. That may mean that this composition is not suitable for making a substrate having the desired properties.